Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1172 Chapter 21: Development of Multicellular Organisms
sensory
mother cell
dying cell
socket cell
shaft cell
sheath cell
neuron
mechanosensory
bristle
Figure 21–34 The basic structure of a
mechanosensory bristle. The lineage of
the four cells of the bristle—all descendants
of a single sensory mother cell—is shown
on the left. The sensory mother cell, once
it is specified, generates this set of cells
through a short program of division cycles.
In each generation of the progeny, lateral
inhibition operates again to drive the
newborn cells toward different fates: one
of the ultimate progeny will become the
neuron; another, the shaft of the bristle;
others, supporting cells of various sorts.
As the sensory mother cell and its progeny
divide, certain proteins are allocated
preferentially to one of each pair of
newborn sister cells, biasing the outcome
of the lateral-inhibition competition
mediated by Notch signaling.
earlier, lateral inhibition mediated by Notch signaling is crucial for both cell diversification
and fine-grained patterning in an enormous variety of tissues in all animals.
One example is the development of sensory bristles in Drosophila, most easily
seen on the fly’s back, but also present on most of its other exposed surfaces. Each
of these is a miniature sense organ, consisting of a sensory neuron and a small set
of supporting cells. Some bristles respond MBoC6 to m22.58/22.34
chemical stimuli, others to mechanical
stimuli, but they are all constructed in a similar way (Figure 21–34). The proneural
genes Achaete and Scute mentioned earlier mark the patches of epidermis
within which bristles will form. Mutations that eliminate the expression of these
genes at some of their usual sites block development of bristles at just those sites,
and mutations that cause expression in abnormal sites cause bristles to develop
there.
The initial cells expressing the proneural genes are called proneural cells, and
they are primed to take the neurosensory pathway of differentiation, but which
of the cells will actually do so depends on competitive interactions among them.
In the first round of these interactions, a single cell within each small group of
proneural cells is picked to serve as the progenitor of the bristle. This single cell is
called the sensory mother cell. It becomes distinct from the other cells of the cluster
through lateral inhibition mediated by the Notch signaling pathway. This operates
in the way we discussed earlier. The cells in the proneural cluster initially all
express both the transmembrane receptor Notch and its transmembrane ligand
Delta, along with proteins that regulate the signaling activity of Delta. Wherever
Delta activates Notch, an inhibitory signal is transmitted that diminishes the tendency
of the Notch-activated cell to specialize as a sensory mother cell.At first,
all the cells in the cluster inhibit one another. However, receipt of the signal in a
given cell diminishes that cell’s ability to fight back by delivering the inhibitory
Delta signal in return. This creates a competitive situation, from which a single
cell in each cluster—the future sensory mother cell—eventually emerges as winner,
sending a strong inhibitory signal to its immediate neighbors but receiving
no such signal in return (Figure 21–35). If a cell that would normally become a
sensory mother cell is genetically disabled from doing so, a neighboring proneural
cell, freed from lateral inhibition, will become a sensory mother cell instead.
The sensory mother cell goes through a short program of further divisions to
generate the set of cells that form the final bristle. Notch signaling acts repeatedly
at successive stages in this program to drive the descendants of the sensory
mother cell along different pathways and assign them to their various specialized
fates. However, it does so in conjunction with additional mechanisms that bias
the outcome of the competition mediated by lateral inhibition. Determinants that